10. Music (iTunes Radio/iCloud)

Apple's built-in Music app took 10th place with 42.546 million average unique users, according to Nielsen.

Price: Free

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9. Instagram

iTunes

Whether for a selfie or a picture of your latest meal, Instagram is the king of photo-sharing apps. The app saw 43.944 million average unique users last year, a 34% increase year-over-year, according to Nielsen.

4. Google Play

Sometimes you need an app to download other apps. Google's online store, Google Play, comes installed on most Android phones and snagged fourth place with 84.968 million average unique users.

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3. YouTube

iTunes

YouTube's mobile app is still the best place to watch online videos on your phone. The app saw 88.342 million average unique users last year, resulting in a 26% year-over-year increase in downloads, according to Nielsen.

1. Facebook

iTunes

Facebook's main app was the most downloaded app of the year, with an incredible 118.023 million average unique users. It has also gained some new users since last year, with a 15% year-over-year increase, according to Nielsen.